Directional microphone system



UCL 22, 1957A F. sPANDcK ETAL 2,810,786

DIRECTIONAL MICROPHONE SYSTEM Filed June 6, 1951 --.HEM

111:1:-lllsie 4 w Nnll /IILTIIILIIJI l W wr M nw/ u United States Patent DraEcTroNAL MnnornoNn SYSTEM Friedrich Spandck and Erwin Giilitzer, Karlsruhe (Baden), Germany, assignors to Siemens & Halslre Aktiengesellschaft, Munich and Berlin, German a German corporation Application June 6, 1951, Serial No. 239,104

Claims priority, application Germany .lune 12, 195@ 5 Claims. (Cl. 179-1) This invention is concerned with a directional microphone system especially for use in wireless and in soundlm work. yThe invention may be employed in the recording of speech and music or for direct casting over loudspeakers.

The use of directional microphones is broadly known in connection with work which requires correction of spatial conditions that might aect the recording. The recording sensitivity is in such microphones directed upon the scene of recording. Prior proposals suggest to use for such purposes a plurality of microphones arranged in serial relationship to form a receiver group in the manner of a straight radiation group, that is, to form a socalled microphone line whose length equals one-half wave length of the lowest frequency which is to be transmitted. The resulting line length was in such arrangements on the order of l to 2 rn. Such arrangements are cumbersome, and it is often diticult to dispose the microphones without obscuring the viewing field.

Research has revealed that it is important especially in .the case of recording in echoing auditoriums and the like to record and to transmit the higher frequencies and that it is in many cases sufficient to employ a high directional adjustment only so 'far as the higher frequencies are concerned.

The invention proposes for this purpose a directional microphone system in which a plurality of microphones of a receiver group are disposed in a row or line in the manner of a straight radiation group (microphone line) whose length equals at the most one-half wave length of a median frequency of the frequency fband that is to be transmitted. The microphone line is disposed vertically or slightly inclined with respect to the scene yof recording. The microphones of the line are in such system :arranged for cophasal operation and may be connected in parallel, series, or in mixed circuit relationship. The result is la bundling of the directional characteristics in a plane which extends perpendicular to the microphone line. in 4order to increase the directional characteristics, it will be advantageous if at least some of the microphones of the line exhibit a pronounced directional effect.

lf the system comprises, for example, a plurality of microphones of such directional characteristics, the directional eect of the Iline at low frequencies is given by the characteristics of the individual microphones, while an additional group-directional effect (which -iinds expression in considerably sharper bundling of the sensitivity characteristics) is obtained for frequencies whose one-half wave length is smaller than the length of the line.

It is in many cases advantageous to use an amplifier which exhibits at least at low and median frequencies an amplification rising with the frequency, because the relatively low directional sharpness at low tones causes preferential direct transmission to the audience of low tones above the echoes of the auditorium so that lower amplication is for such frequencies sufficient.

ln the case of a system comprising 'capacitor microice phones, the invention proposes to arrange such microphones on the suitably shaped, for example, generally cylindrical or bottle-shaped, pre-amplifier housing or casing. 'Such casing, together with the microphones carried thereon, is incident to recording suitably so disposed that the microphones are disposed perpendicular for slightly inclined to the scene of recording, and that the sensitivity maxima thereof is trained approximately to the center of the recording scene. By such arrangement is obtained a certain independence of the sensitivity on the distance 'between the sound source and the microphones, since the sound source with 4increasing distance from the line is thereby placed in a region -of greater directional effect and therewith greater sensitivity.

if a sharper directional effect is also desired for low frequencies, the invention, in accordance with another object, contemplates provision of two microphone lines which Iare arranged with their longitudinal `axes in parallel and in serial relationship relative to the direction of the sound. Each microphone line comprises again a plurality of vertically successively disposed cophasally operating microphones and the two microphone lines are electrically connected in opposition to form a pressure gradient system. The lateral spacing between the two lines is such that the rst maximum of the pressure gradient ycoincides approximately with that frequency at which the group-directional effect of a line becomes effective. The arrangement exhibits good directional eect even at low frequencies, and the sensitivity at such'frequencies is by suitable choice of the lateral spacing between the microphone lines sufficient.

In accordance with another feature of the invention, there is provided in serial relationship with one of the two microphone groups or lines a low-pass filter whose upper limit freequency lies in the neighborhood of the first maximum of the pressure gradient. Only one of the two lines is effective above this limit frequency which has, however, a sufticient -directional sharpness, due to the group-directional effect, which becomes manifest.

The foregoing objects and other robjects and features will appear from a more detailed description of the accompanying drawings, in which:

Figs. l and 2 illustrate in diagrammatic manner embodiments in each of which a microphone line is carried by a generally cylindrical or bottle-shaped pre-amplifier casing;

Fig. 3 indicates the manner of interconnecting two microphone lines to form a gradient system; and

Fig. 4 shows diagrammatically an example of two microphone lines each comprising four microphones which may be connected in a circuit :as indicated in Fig. 3.

The microphones 1 of Fig. l form a line which is so arranged relative to an extension of the pre-amplifier casing 2 that its vertically extending longitudinal axis coincides with the axis of the casing, as is indicated by the dot-dash l-ine 3. The line of microphones is for electrical shielding surrounded 'by a hood 4 which may oe a sound-permeable wire netting or the like placed on the upper end of the casing 2. The current is supplied by way of a cable 5 which also connects the pre-amplier with the input of the succeeding amplifier.

ln Fig. 2 the line comprising the microphone 6 is disposed alongside -the pre-amplifier casing 7 in such a manner that its vertically .extending longitudinal axis 8 extends in parallel with the casing axis 9. The microphones are again shielded by a suitable sound-permeable hood 1t?. The current supply and interconnection with the succeeding amplifier are over the cable 11.

The mounting of the microphones of Figs. l and 2 may be so constructed and arranged that it remains acoustically ineffective, for example, by the provision of secur- 3 ingv rings Vwhich may be thin rods, Iinterconnected with each other and vwith vthe casing.

The microphones of a line may he 4arranged upon a small, narrow, elongated soundboard which will advantageouslyA 'affect the directional operati-on. Such .soundboard, indicated in Figs. 1 and 2 at 'SB may .be included in each example shown. Y

The interconnection of two microphone lines to form a4 pressure gradient system is apparent from Fig. 3. The two lines areV indicated in the drawing, each by a microphoneshown at 12and 13,` respectively.. yin this schematic representation, the more prominently illustrated electrode denotes the counterele'ctrodes. of the microphones of a. line which may, for example, 'hefconne'cted in. parallel. Themicrophones ofv both lines 12' and 1'3 lie on their alternating current' sides in parallel over thev resistor. 14, on Y--A of a control grid. of'an' amplifier tuhel'.V The biasv of' this tube produced. at the cathode. resistor la'lfwhieh bridged. a capacitor 16t The bias. for the' microphones is obtained; from the anodeV voltage. +A. In the; case. of. the .microphones 125. the positivepole of. the bias: lies onthe'diaphragmsf, andin the-case of the microphones 1? it lies on the counterelectrode.V Thev direction of vthe electriceld therefore. is in the two microphones (lines)A opposite and; the: differential of the Vmicrophone voltages thus becomes eective on' the grid of the tube 115. This is necessary for obtaining: the. desired gradient effect.V Short-circuiting of the microphone voltage to A over the resistor 14 is prevented'. by capacitor 1S'. The

high-ohmic resistor 19 prevents ashuntingof the yalternatingj voltage of the microphones 13.I over the net. -Between the vcounterelectrodesof the microphones 13 Vand the capacitor 18.v disposed. a; low-pass Vlil-ter 20 which Y may, for example, comprise a.. choke andl a. resistor parallel' thereto. The low-'pass filter 20:makes the microphones 13` ineffective for frequencies .for whicl'iV Va group-directional eiect is obtained in the line represented by the micro phoneV 12'.

The entire arrangement therefore operates at lowfrequencies as a pressure' gradient system and at median and Vhigh. frequencies as a simple line. system. The output voltageU'is-tapped at; the anoderesist-or 21 of the tube 15.

Fig. 4 shows. by way of" example two microphone lines which'm-ay be conn-ected ina' circuit as indicated in'V Fig. 3, each line including foury microphones 22"-23', respectively.' The two lines are soarranged on a horizontally extending pre-amplifier casing 24 thattheir respective verticallyextending longitudinal axes 2'S-Z6are perpendicular to the axis ofY the casing 24. The directional sensitivity'maxima-of'bothlines.are such thatLeach-line yields a maximum initial, voltage nesponsive to the sound input in the direction of'thev arrow 2S; With. identical sensitivity'oi both lines, there will be obtained, i'n view ofr'their connection. in opposition, ya resultingfoutput'voltage which of a length which is at the length of a frequency range which is to be transmitted, circuit means for connecting said microphones in each of said groups for cophasal operation, each group of microphones causing a bundling lof the directional characteristics in Ia plane which extends perpendicular to the plane of the vertical line formed thereby, and means for electrically connecting said microphone groups in oppo.-

sition to form a pressure gradient system,fthe. spacingbetween sai-d two groups of microphonesheing suchthatthe firs-t maximum of the pressure gradient coincides approximately with the frequency at which the group-directional effect of one of said microphone groups becomes. effective.

2. A directional microphone system comprising. :two separate groups of microphones each group comprising a plurality of individual microphones, means for disposing the microphones of said two groups with their longitudinal axes in parallel, thermicrophonesof. each group extending successively along a Substantially vertical line of. a length which is at the most'equalf tonne-half wavey length of. a frequency range which. is tofbe` transmitted,.-

circuit means for connecting `said microphones in. each of said groups for cophasal operation, each group of microphones causingl a bundling' of thedirectional characteristics inV a'plane which extends'perpendicular to the plane of the vertical line Vformed thereby, meansfor elec- .trically connecting said microphone groups in YoppositionV to formv a pressure gradient system, and. al low-pass lter connected Vserially with one of said. microphone groups, the. upper limit frequency ofv said .lter being at that frequency at which the.group-directionalA effect of the other groupof microphones becomes'reiect-ive. Y '4 Y 3. ln the art of recording sound, directional micro phone means for receiving said sound, said. microphone means comprising :a plurality of microphones, `amplifier means for Vamplifying said.` sound, a. casing: for said' amplier means, means for securingsaid microphones on. said casing in agenerally vertically extending line and spaced fromv said casing, circuit means. for connecting saidmicrophones for cophasal operation, and means` forming a directional lsoundboardV for said microphones, said micro-- phones causing a bundling of the directional characteristicsr in av plane which extends perpendicular to thefline formed thereby. Y

4.V='I`he arrangement as-defmed in claim 3, wherein said soundhoard isV disposed 'between said microphones and saidv casing. Y g

5..Y The; arrangement. as` dened in claim 3, wherein; the length ofV they line. formed by said microphones; is substantially equal toene-halffwave length lof a median*A frequency of the frequency bandto be recorded.

References.CitedinY the leof this patent Y UNITED'. STATESY PATENTS 1,422,877 Maxfield July 18, 1922 1,755,484 Mar-k Y -Y Apr; 22,1930 1,855,149 Jones Apr. 19,v 1932 1,897,222 WeinbergerV Feb. 14, 1933 1,975,283 Mueller Oct. 2; 19.3-4 1,988,001 Flanders Jan. 15, 1935 2,173,219 Anderson Sept. 19, 1939 Y 2,301,744 O1son. Nov. 10,v 1942 V2,305,599 j Bauer Dec. 22, '1942 2,309,109 Hathaway Ian. 26 1943 v2,396,222 Foldy ..Y Mar. '5., 1946 2,396,519 lMassa- Mar. 12, 1946 most equal to one-llalfwaveV Y 

